The invention relates to a data receiver comprising:
an input part for converting a transmission signal into a digital baseband signal having samples relating to a distinct bit in said transmission signal; PA1 a metric calculator for calculating, from each of the digital baseband signal samples, a metric comprising a gross bit representing the value of said distinct bit and at least one reliability bit for indicating the reliability of said gross bit; PA1 a soft-decision decoder for deriving data from metrics supplied thereto. PA1 to a first extreme value (for example, all metric bits are `0` or `1`), in response to a digital baseband signal sample whose value is smaller than a first saturation value; PA1 to a second extreme value (for example, all metric bits are `1` or `0`), in response to a digital baseband signal sample whose value is larger than a second saturation value; and PA1 to a value according to a monotonous transition function between the extreme values, in response to a digital baseband signal whose value is in a range bounded by the first and the second saturation value, said range comprising a number of 2.sup.k different digital baseband signal sample values including the boundary saturation values, k being an integer larger than the number of reliability bits (r) but smaller than or equal to the number of bits (n) in the binary representation of the digital baseband signal.
The invention also relates to a method of calculating metrics from a digital baseband signal in a data receiver and to a signal-processing device for performing said method.
Such a receiver is known from `Specification, Partitioning and Design of a DAB Channel Decoder` by J. A. Huisken et al., VLSI Signal Processing, volume VI, pages 21 to 29, IEEE Special Publications October 1993, in which a receiver for Digital Audio Broadcasting (DAB) is described. The known receiver successively comprises a front-end, a quadrature demodulator, a channel decoder and an audio decoder, which is shown in the block diagram of FIG. 1 of the cited document. The front-end and quadrature demodulator retrieve and subsequently digitize the complex modulation signal from an analog RF DAB-signal at the receiver input. The digitized complex modulation signal, having real and imaginary signal components, is supplied to the channel decoder. From this signal, the channel decoder retrieves data which is fed to an audio decoder.
The channel decoder performs signal processing inverse to that of the channel encoder at the transmitter end. Channel encoding and decoding at the transmitter and receiver end respectively, like in DAB, is used to enhance the robustness of the transmission. Transmission channel deficiencies may affect the transmission signal in the sense that bit errors are introduced. The channel decoder in the known receiver comprises a Viterbi decoder for correcting bit errors by using redundancy in the DAB transmission signal. At the transmitter end this redundancy has been added by Viterbi encoding of the data to be transmitted.
The Viterbi decoder in the known receiver is of the soft-decision type, well known to those skilled in the art. Generally stated, a soft-decision decoder may retrieve transmitted data at a lower bit-error rate (BER) than a hard-decision decoder under similar reception conditions. This is because a soft-decision decoder can use redundancy in a transmission signal more effectively than a hard decision decoder. As it were, a soft-decision decoder concentrates its error-correcting capabilities on those bits which are most likely erroneous. Consequently, the soft-decision decoder requires input data also comprising bit reliability indicators so as, to distinguish the more and the less "certain" bits.
In the known receiver, the data elements input to the soft-decision decoder are referred to as metrics. A metric comprises a gross bit and three reliability bits. The gross bit indicates the most likely value of a bit in the transmission signal to which the metric relates. The reliability bits indicate the error probability of this gross bit. It is to be noted that a number of three reliability bits is a mere design choice. It would equally have been possible to use a different number of reliability bits in a metric.
In the known DAB receiver, the metrics are calculated in the demodulation processor from a digital baseband signal having samples which relate to bits in the transmission signal. Said digital baseband signal is obtained by further processing of the digitized complex modulation signal supplied by the quadrature demodulator. It comprises Fast Fourier Transform (FFT) add differential phase calculations on successive groups of digitized complex modulation signal samples. This further processing is inverse to the processing at the transmitter end relating to the modulation technique as used in DAB: Orthogonal Frequency Division Multiplex (OFDM). With this modulation technique, the bits in the transmission signal are represented as phase differences in a frequency multiplex of carriers.
It has been proposed to jointly perform said further processing and metric calculations by means of a commercially available digital signal processing device, for example the TMS 320. The versatility of these types of devices renders them very well suitable for use in experimental receivers, the signal-processing characteristics being readily adjustable by reprogramming software stored in a memory within the device or coupled thereto. However, the use of these devices in mass-produced consumer products is generally not very cost-effective. It has therefore been proposed to develop a demodulator processor in dedicated hardware.